The present invention relates to a catalyst based on rare earth metal compounds, to a process for the production thereof and to the use thereof for the polymerization of conjugated dienes.
It is known to polymerize conjugated dienes with the assistance of catalysts based on rare earth metal compounds, for example, with compounds of cerium, lanthanum, praseodymium, neodymium or gadolinium. In such a case, the catalysts based on rare earth metals preferably comprise a salt of a rare earth metal, an organoaluminum compound as co-catalyst and a compound capable of liberating halogen.
In this connection, WO-A 93/05083 is referred to, which describes a process for polymerizing conjugated dienes by means of a catalyst based on rare earth metal compounds.
A disadvantage of the process described in the above-patent publication is the presence of the catalyst used therein, which is based on rare earth metal compounds using silicon halides or organosilicon halides as the halogen-providing compound. Such catalyst has low activity and the low rate of polymerization (conversion) achieved therewith.
An object of the present invention is to avoid the disadvantages of the catalyst described in WO-A 93/05083 in the polymerization of conjugated dienes and to provide a catalyst system based on rare earth metal compounds, which in particular provides an elevated space-time yield, a low polymer solution viscosity and a high molecular weight polybutadiene having excellent processing properties which exhibits improved dynamic and mechanical properties in the vulcanized state.
The object of the present invention has been achieved by a catalyst system based on rare earth metal compounds in that specific trihalosilyl compounds are used as one component of the catalyst system.
Accordingly, the present invention provides a catalyst system based on rare earth metal compounds which comprises
a) a rare earth metal compound,
b) an organic aluminum compound and
c) a trihalosilane of the formula 
xe2x80x83wherein
Hal represents fluorine, chlorine or bromine and
R represents hydrogen or a vinyl group,
in which components a):b):c) are present in a ratio by weight of 1:0.5-5:0.05-0.5.
According to the present invention, preferred catalyst systems based on rare earth metal compounds are those in which components a):b):c) are present in a ratio by weight of (1):(1-2):(0.1-0.4).
Preferably used for component a) are rare earth metal compounds wherein the rare earth metal compounds are soluble in hydrocarbons. Corresponding rare earth metal salts are more preferably used.
Rare earth metals which may be mentioned as being particularly suitable for the synthesis of component a) are cerium, lanthanum, praseodymium, gadolinium and neodymium, wherein neodymium is more preferred.
Hydrocarbon-soluble salts which may be mentioned are neodymium carboxylates, preferably neodymium neodecanoate, neodymium naphthenate, neodymium octanate, neodymium 2,2-diethylhexanoate, neodymium 2,2-diethylheptanoate and the corresponding salts of lanthanum or praseodymium. Neodymium neodecanoate is most preferred.
The rare earth metal compounds may be used both individually and as mixtures with each other, wherein the most favorable mixture ratio may readily be determined by appropriate preliminary testing.
Organic aluminum compounds (component b)) which may be considered are the alkylaluminum compounds and alkylaluminum hydrides known for such catalyst systems, in which the alkyl group has 1 to 10, preferably 1 to 6, carbon atoms. The alkylaluminum hydrides may have one or two alkyl groups. Preferred compounds which may be mentioned are triethylaluminum, diisobutylaluminum hydride, triisobutylaluminum, more preferably diisobutylaluminum hydride.
Trihalosilyl compounds (component c)) which are used in the catalyst system according to the present invention are preferably tribromosilyl hydride, trichlorosilyl hydride, vinyltribromosilane and vinyltrichlorosilane, most preferably trichlorosilyl hydride.
Most preferred catalyst systems based on rare earth metal compounds are those which comprise
a) neodymium neodecanoate
b) diisobutylaluminum hydride and
c) trichlorosilane
wherein components a):b):c) are present in a ratio by weight of (1):(1-2):(0.1-0.4).
The present invention also provides a process for the production of the catalysts according to the present invention based on rare earth metal compounds comprising the above-stated components a), b) and c), wherein the process is characterized in that components a), b) and c) are brought into contact with each other at temperatures of xe2x88x9230 to 100xc2x0 C., preferably of 20 to 80xc2x0 C., in the above-stated quantity ratios, which means that components a):b):c) are used in a weight ratio of (1):(0.5-5):(0.05-0.5), preferably of (1):(1-2):(0.1-0.4).
The present invention also provides for the use of the catalyst system according to the present invention based on rare earth metal compounds comprising components a) to c) described above for polymerizing conjugated dienes.
The polymerization of conjugated dienes by means of a catalyst system based on rare earth metal compounds is known per se and is conventionally performed in such a manner that the dienes to be polymerized are dissolved in a suitable hydrocarbon, for example, hexane, and the catalyst components are then added.
Care must, in particular, be taken to ensure that the catalyst components, the solvent and the monomer(s) are inert, i.e. anhydrous and oxygen-free.
It is, of course, also possible to polymerize the conjugated dienes in the absence of or only in the presence of small quantities of solvents.
The catalyst system according to the present invention is preferably added to the dienes to be polymerized in a concentration such that component a) (rare earth metal compound) is present in a quantity of 0.01 to 0.2 wt. % of the diene monomers. The optimum quantity of catalyst system to be used may vary and depends upon the particular reaction conditions and upon the desired molecular weight of the polymer.
Conjugated dienes which may be polymerized with the catalyst system according to the present invention and may, in particular, be mentioned are butadiene, isoprene, piperylene, 1,3-hexadiene, 1,3-octadiene, and 2-phenyl-1,3-butadiene. The conjugated dienes may, of course, also be copolymerized as mixtures with each other. It is preferred to produce polybutadiene, polyisoprene and isoprene/butadiene copolymers.
The polydienes produced with the catalyst system according to the present invention may be used in the production of rubber articles of all kinds, preferably for the production of tires or golf balls. It is also possible, especially in the case of polybutadiene, to use the latter for the production of high impact polystyrene (HIPS).
With regard to the known prior art, it is particularly surprising that, by using the catalyst system according to the present invention based on rare earth metal compounds, in which specific trihalosilyl compounds are used, it is possible to obtain polydienes at elevated space-time yields, which are in particular distinguished by a particularly high molecular weight associated with good processing properties and improved dynamic and mechanical properties in the vulcanizates.